This invention relates generally to the tracking of the transverse motion of an aperture pattern in a longitudinally moving strip of material and particularly to the sensing of the longitudinal edges of the aperture pattern in the shadow mask web during the production of kinescope shadow masks.
In the manufacture of shadow masks for color television kinescopes a roll of flat material is coated with a photoresist material and is subsequently photoexposed to form a series of aperture patterns and the peripheries of the shadow masks in the photoresist material. The unexposed photoresist material is washed away leaving bare metal. The strip of material is subjected to an acid etching process in which the bare metal is removed to form the apertures and partially etched peripheries used to remove the shadow masks from the strip of material. After the etching is completed the transmission of energy through the apertures is measured to verify that the shadow masks are suitable for the intended use. The measurement of light transmission through the apertures typically is accomplished by shining a known intensity of light through the apertures and noting the percentage of light which passes through the apertures. The acid etching and the light transmission measurements typically are made while the strip of material is pulled longitudinally along a conveyor line. Because of the long length of the strip, there is a tendency for the strip to move transversely back and forth perpendicular to the longitudinal motion. The areas of the apertures varies along the transverse dimension of the pattern and for this reason, accuracy of the light transmission measurement requires that all measurements be made at the same transverse position of the aperture pattern. Accordingly, the transverse motion causes a degregation of the measurement accuracy. It is extremely difficult, if not impossible, to restrain the long strip and prevent the transverse motion. For this reason, there is a need for a system for tracking the transverse motion of the strip so that the measuring densitometer maintains a constant relative transverse position with respect to the strip. Accordingly, there is a need for a device for tracking the longitudinal edges of the aperture patterns as a technique of sensing the transverse motion. The instant invention fulfills this long felt need.